Phase-based modulation techniques such as the differential binary phase-shift-keying (DBPSK) and differential quadrature phase shift key (DQPSK) have received significant attentions in recent years due to their superior tolerance to amplified spontaneous emission (ASE) noise and fiber nonlinear impairments as compared to traditional intensity-based modulations techniques such as non-return-to-zero (NRZ), return-to-zero (RZ), carrier suppressed return-to-zero (CSRZ) and others as known by those skilled in the art. Advanced modulation techniques have become a major enabler for long-reach wavelength division multiplexing (WDM) transmission at 40 Gb/s or above data rates. Core network trends in the industry for longer signal transmission distance (thereby reducing signal regeneration) and higher bit rate have resulted in large scale deployment of remotely re-configurable optical add/drop multiplexers (ROADM) to bring signals on and off the backbone. Deployment of dynamic optical networks (i.e. optical routing networks) increases operating efficiency and also presents new challenges. For example, in an optical routing network, the optical signals will pass through successive optical filters as they are transported to and through each of the ROADMs along the route. Additionally, signals with a different wavelength or the same wavelength at a different time period (phase) may travel on different routes. Because of the differing routes, the signals may experience different optical filtering as the number of ROADMs along different routes will vary.
For differential-phase modulated optical communication system, a delay-interferometer (DI) with a free-spectral range (FSR) equal (approximately) to the symbol rate is commonly used as the demodulator. When no optical filtering is employed, it is well known by those in the art, that using a demodulator with DI FSR equal to the symbol rate typically yields optimal performance (lowest losses). When strong optical filtering is employed, as in the case where 40 Gb/s signals travel through multiple 50 GHz-spaced ROADMs, the performance of DBPSK-based WDM systems can be significantly improved by using a demodulator with DI FSR greater than the typical symbol rate (approximately equal to the bit rate for DBPSK).
In an optical routing network, the optimal DI FSR settings for different wavelengths or the same wavelength at a different time period (phase) can be quite different, largely due to different filtering conditions. For example, using a traditional demodulator with ‘fixed’ DI FSR (the tunable range for FSR is very small and therefore considered negligible) for all wavelengths, only some of the wavelengths will yield optimal transmission performance, i.e. lowest inter-symbol interference (ISI). Consequently, the transmission performance of many other wavelengths can be severely degraded due to non-optimal demodulator setting as an inevitable consequence of a design utilizing a ‘fixed’ DI FSR and an assumption regarding the amount of filtering. An additional drawback to this design is a significant optical signal-to-noise ratio (OSNR) penalty if the deployed network configuration and wavelength routing do not experience the assumed worst case filtering effect.
FIG. 1 shows a prior art schematic illustration 100 of a differential phase-modulated optical communication system as commonly used in optical routing networks. In this specific example there is shown a plurality of nodes, A 102, B 104, C 106, D 108 and E 110. Optical signals at four distinct wavelengths λ1, λ2, λ3 and λ4 are transmitted from node 102 (the source node) to nodes 104, 106, 108 and 110 (the destination nodes). The signals are generated per known differential-phase modulated optical signal generation techniques such as DBPSK or DQPSK, as known by those skilled in the art. Along the signals' path from source to destination node, they will pass through a single wave division multiplexer (WDM) 112, multiple optical amplifiers (OAs) 114 and one or more remotely re-configurable optical add/drop multiplexers (ROADMs) 116. Each destination node, 104, 106, 108 and 110, is comprised of two components, a demodulator 118 and a detection unit 120. As shown in FIG. 1, λ1 will pass through one ROADM 116 as it travels from node 102 to node 104, while λ2 will pass through two ROADMs 116 as it travels from node 102 to node 106, and λ3 and λ4 will pass through three ROADMs 116 as they travel from node 102 to nodes 108 and 110, respectively. Inside each ROADM 116 are optical filters which attenuate part of the spectral component of the signal. As signals are successively passed through ROADMs 116 they are increasingly filtered, yielding a stronger optical filtering effect. In this example, different wavelengths will see different filtering conditions. With the differential phase-modulated optical communication system illustrated in FIG. 1, the free-spectral range (FSR) of each of the plurality of demodulators 118 is fixed to be around the symbol/baud rate. As a result of the filtering effect on optical signals as they are passed through successive ROADMs 116 as discusses above, it will be obvious to those skilled in the art that only λ1 may operate at a point that is close to the optimal performance point while λ3 and κ4 will definitely operate at non-optimal levels.
In FIG. 2 is shown a prior art schematic 200 illustration of common (a) DBPSK and (b) DQPSK demodulators. For both demodulators, the path delays 202 are fixed to be close to the symbol period (Ts)/baud rate while a tunable phase shift 204 is introduced to achieve the required phase difference between the two paths. For the DBPSK technique the tunable phase shifter 204 is typically set to 0 or π, while for the DQPSK technique it is typically set to ±π/4. The tunable phase shift 204 is also used for frequency tuning as well as small delay fluctuation compensation. The phase difference 204 between the two paths depends on the optical frequency as well as the time delay Δt, which in this case is close to the symbol period (Ts)/baud rate.
It would therefore be desirable to significantly improve the performance of DQPSK-based WDM systems by using a demodulator with DI FSR greater than the typical symbol rate (approximately equal to half the bit rate for DQPSK) when strong optical filtering is applied as is the case when 100 Gb/s signals go through multiple 50 GHz-spaced ROADMs. Additionally, it would be desirable to substantially suppress inter-symbol interference (ISI) caused by optical filtering by optimizing/adjusting the demodulator FSR setting for both DBPSK and DQPSK-based communication systems.